1. Field of the Invention
The present invention relates to current-collecting thin composite plates for fabricating compact fuel cell stacks and to fuel cells fabricated using such a composite plate. In particular, the present invention relates to current-collecting composite plates for fuel cells with excellent corrosion resistance, excellent durability and low internal resistance and to fuel cells with a high power generation efficiency fabricated using such a composite plate.
2. Description of Related Art
Fuel cells have high conversion efficiency because they directly convert chemical energy into electrical energy. Also, they do not burn a fuel containing nitrogen (N), sulfur (S), etc. and therefore are environmentally friendly because they emit less air pollutants (such as NOx and SOx) Examples of such fuel cells include polymer electrolyte fuel cells (PEFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs). Among these, PEFCs are expected to be widely used in the future as power sources for automobiles, homes, mobile devices, uninterruptible power supply systems, etc.
FIG. 1 is a schematic illustration showing a mechanism of power generation in a fuel cell using methanol as a liquid fuel. This type of a fuel cell is called a direct methanol fuel cell (DMFC). As shown in FIG. 1, in a DMFC 71, a mixture of methanol fuel and water is supplied to a fuel electrode 72 where it produces hydrogen ions and carbon dioxide (CO2) gas with the aid of a catalyst. The hydrogen ions migrate through a polymer electrolyte membrane 73 to a counter electrode as an oxidant electrode. Then, on an air electrode (oxidant electrode) 74, electrons generated by the ionization, oxygen as an oxidant and the hydrogen ions react to produce water. These sequential reactions allow generation of electric power, whereby electrical energy can be taken out from the fuel cell.
The liquid fuel and air (oxidant gas) are each supplied to the corresponding electrode via a passage comprising a channel which allows the respective substances to pass through. These passages also function to vent water and gases generated during power generation.
FIG. 2 is a schematic illustration showing a cross-sectional view of a conventional DMFC unit cell 81. It includes: an MEA 82 (comprising: a solid polymer electrolyte membrane 84; a fuel electrode 83 provided on one surface of the membrane 84; and an air electrode (oxidant electrode) 85 provided on the other surface of the membrane 84); a metal bipolar plate 87 facing the fuel electrode 83 of the MEA 82 and having multiple fuel passage conduits 86 on the side facing the MEA 82; a metal bipolar plate 89 facing the air electrode 85 of the MEA 82 and having multiple air (oxidant gas) passage conduits 88 on the side facing the MEA 82; and a gasket 90 provided between the bipolar plates 87 and 89 for sealing the perimeter of the MEA 82. Typically, a plurality of such fuel cells 81 are stacked in order to increase the power output. A bipolar plate is sometimes also called a “separator”.
FIG. 3 is a schematic illustration showing a stack structure of a conventional fuel cell 91 using current-collecting plates (bipolar plates) 92. In the conventional fuel cell 91, the fuel electrode (i.e., anode, shown as “−” in FIG. 3) and the air electrode (i.e., cathode, shown as “+” in FIG. 3) are disposed alternately, i.e., in series.
DMFCs are expected to be used for compact sized mobile devices, which use a secondary battery at present, because it can take out electrical energy by using methanol as a liquid fuel, and it has been practically used in some areas. On the other hand, recently, PEFC using hydrogen gas as a fuel has been intensively investigated to be used for automobiles. In the PEFC, to supply hydrogen gas, a reformer is used to produce hydrogen containing gas from, e.g., methanol or natural gas.
In contrast, the DMFC has a possibility that its cell system can be considerably downsized because it is capable of taking out hydrogen ions directly from methanol. However, since the DMFC has a lower output density than the PEFC using hydrogen gas as a fuel, the application of DMFC is limited to devices with low electric power consumption at present. In the DMFC, other liquid fuels than methanol such as dimethylether can be used, and the practical use of each liquid fuel has been studied (see, e.g., JP-A-2002-175817).
The above JP-A-2002-175817 discloses a fuel cell of DMFC that a passage is formed to exhaust carbon dioxide (CO2) gas produced during the power generation on its fuel electrode side so that an equipment for gas-liquid separation becomes unnecessary, whereby the DMFC system can be simplified and be downsized. However, turning again to FIG. 3, the fuel cell stack 91 as a conventional fuel cell has a disadvantage in that the fuel and oxidant need to be separately supplied to respective ones of adjacent unit cells (respective opposite surfaces of a bipolar plate) to prevent mixing of the two fluids, thereby potentially making the configuration of the fuel and oxidant supply and exhaust lines more complicated.
JP-A-2006-31963 discloses a membrane-electrode-assembly (MEA) module and a fuel cell aimed at downsizing a DMFC and simplifying a DMFC system. FIG. 4 is a schematic illustration showing a perspective view in which a conventional compact fuel cell is being assembled in a portable terminal; FIG. 5 is a schematic illustration showing a cross-sectional view of a structure of the conventional compact fuel cell shown in FIG. 4; and FIG. 6 is a schematic illustration showing an expanded perspective view of the conventional compact fuel cell shown in FIG. 4. As shown in FIGS. 5 and 6, in the MEA module 111, a current-collecting plate 113 with a film is folded around supporting rods 112 provided along valley lines v to tightly sandwich the MEAs between adjacent portions of the plate 113.
Such an MEA module is accommodated in a casing 115 having a fuel tank 114 to assemble a fuel cell 101 (shown in FIG. 5), which is then incorporated into a portable terminal P as shown in FIG. 4. The above JP-A-2006-31963 says that the MEA module may be applicable not only to DMFCs but also to PEFCs.
However, the technologies of the compact fuel cell described above do not offer a solution concerning corrosion resistance, durability; and internal resistance of the current-collecting plate. In the fuel cell stack 91 shown in FIG. 3, the current-collecting plate 92 is called the “bipolar plate”, which functions to collect and carry current as well as functioning to separate the flowing of the fuel and oxidant gases along respective opposite surfaces thereof. Materials usable in such current-collecting plates 92 (bipolar plates) are being extensively studied for PEFC and DMFC applications.
For example, there is a proposed technique in which a base of dense carbon or stainless steel is plated with a 0.01 to 0.06 μm thick noble metal (e.g., JP-A-2001-93538). Also, another technique is described in which a cladding material of a corrosion resistant Ti-based metal is further covered with a contact layer having good electrical conductivity and good corrosion resistance (e.g., JP-A-2004-158437).
Generally, a fuel cell using a bipolar plate (current-collecting plate) formed of a SUS without surface treatment has a problem because constituent metals of the SUS can dissolve into the ambient environment and degrade the properties of the catalysts and electrode membrane of the fuel cell, thereby resulting in an extremely shortened service life. Also, when a SUS or a Ti-based cladding material is used for a fuel cell bipolar plate, the surface oxide film formed on such a metallic material can increase the electrical contact resistance and therefore increase the internal resistance of the fuel cell. As a result, some sort of electrically conductive surface treatment is needed.
In order to obtain a thin and compact fuel cell stack, the material used for the current-collecting plate is practically limited to metallic materials. Thus, the fuel cell stack 101 as shown in FIG. 4 also has similar problems of what core metal to employ and what optimum surface treatment to apply. On the other hand, the current-collecting plate 113 of the fuel cell stack 101 is configured so that the multiple unit cells are interconnected by surface wiring lines. Thus, the current-collecting plate 113 using a corrosion resistant metallic core (such as Ti and SUS) treated with an electrically conductive surface treatment still has a problem since the corrosion resistant metallic core has a relatively high electrical resistivity, and thereby the internal resistance of the plate 113 is prone to increase with increasing the current path length.